Make Site#configure_theme more understandable

lib/jekyll/site.rb

@@ -423,15 +423,14 @@ module Jekyll
 
     private
     def configure_theme
-      self.theme = nil
-      return unless config["theme"]
-
-      if config["theme"].is_a?(String)
-        self.theme = Jekyll::Theme.new(config["theme"])
-      else
-        Jekyll.logger.warn "Theme:",
-          "value of 'theme' in config should be String, but got #{config["theme"].class}"
-      end
+      self.theme =
+        if config["theme"].is_a?(String)
+          Jekyll::Theme.new(config["theme"])
+        else
+          Jekyll.logger.warn "Theme:", "value of 'theme' in config should be "
+          "String to use gem-based themes, but got #{config["theme"].class}"
+          nil
+        end
     end
 
     private